Sema: Build the AST for inout address conversions.

Add two new AST node types:

- InOutConversionExpr, which represents an '&x' expression that involves inout conversion. This will be a signal to SILGen not to introduce a writeback scope for the nested conversion call.

- LValueToPointerExpr, which represents the primitive '@lvalue T' to 'RawPointer' conversion that produces the argument to the inout conversion.

Build an InOutConversionExpr AST when an inout expression is resolved by a conversion to an BuiltinInOutAddressConvertible type.

Swift SVN r15594

lib/Sema/CSGen.cpp

@@ -1191,6 +1191,9 @@ namespace {
     Type visitOpenExistentialExpr(OpenExistentialExpr *expr) {
       llvm_unreachable("Already type-checked");
     }
+    Type visitInOutConversionExpr(InOutConversionExpr *expr) {
+      llvm_unreachable("Already type-checked");
+    }
   };
 
   /// \brief AST walker that "sanitizes" an expression for the